18 (1410U30-1) Examiner only 11. Chlorine can form a range of oxyanions with chlorine present in oxidation states including +1, +5 and +7. (a) Chlorate(I) ions, ClO–, are commonly present in household bleaches. These ions can oxidise iodide ions to iodine. © WJEC CBAC Ltd. H2O I2 CIO– 2OH– CI– 2I– + + + + Dewi took a 25.0 cm3 sample of a household bleach and diluted it to form 250 cm3 of solution in a standard volumetric flask. Four 25.0 cm3 portions of the diluted solution were measured and placed in separate conical flasks and a spatula of solid potassium iodide was added to each. Each portion was titrated against standard sodium thiosulfate solution from a burette. 1 2 3 4 Volume of Na2S2O3 solution / cm3 6.45 6.40 6.50 6.45 (i) Calculate the mean volume of sodium thiosulfate solution used. [1] Mean volume = ....................................................... cm3 2S2O3 2– I2 + S4O6 2– 2I– + 18 Dewi had access to five concentrations of sodium thiosulfate: 2.00 mol dm–3, 1.00 mol dm–3, 0.500 mol dm–3, 0.200 mol dm–3 and 0.0500 mol dm–3. He selected the 0.500 mol dm–3 solution and his results are shown in the table. (1410U30-1) Turn over. 19 Examiner only (ii) Find the concentration of sodium chlorate(I) in the initial bleach sample. [3] Concentration = ....................................................... mol dm–3 (iii) Bleach concentrations are often quoted on labels as % w/v, that is the mass of sodium chlorate(I) dissolved in 100 cm3 of water. Calculate the % w/v concentration of sodium chlorate(I) in this bleach. [1] Concentration = ....................................................... % w/v (iv) The teacher tells Dewi that he has selected an inappropriate concentration of sodium thiosulfate solution. Suggest which concentration of sodium thiosulfate he should have chosen. Give two reasons for your answer. [2] © WJEC CBAC Ltd. 19 20 (1410U30-1) Examiner only (b) Chlorate(V) anions, ClO3 –, are strong oxidising agents and can oxidise bromide ions in acid solution to form bromine and chloride ions. © WJEC CBAC Ltd. 3H2O CIO3 – CI– 6Br 3Br2 6H+ + + + + – The initial rate of this reaction was measured at a temperature of 298 K using different concentrations of reactants. (i) I. Find the orders of reaction with respect to chlorate(V) and bromide ions. [2] You must show your working. Order with respect to chlorate(V) ions ....................................................... Order with respect to bromide ions ....................................................... Experiment Concentration of ClO3 – / mol dm–3 Concentration of Br– / mol dm–3 pH Initial rate / mol dm–3 s–1 1 0.30 0.20 0 3.06 × 10–7 2 0.60 0.20 0 6.12 × 10–7 3 0.30 0.60 0 9.18 × 10–7 4 0.30 0.60 1 20 (1410U30-1) Turn over. 21 Examiner only II. The student finds that the rate of reaction is proportional to [H+]3. This is described as third order with respect to hydrogen ions. Find the expected initial rate for experiment 4. [2] Initial rate = ................................................................................... mol dm–3 s–1 (ii) Chemists often use an approximate rule that the rate of a reaction doubles when the temperature is increased by 10 °C. Show that this rule is true if this reaction is warmed from 298 K to 308 K. The activation energy for the reaction is 52.8 kJ mol–1. [3] © WJEC CBAC Ltd. 21 22 (1410U30-1) Examiner only (c) The acid formed from chlorate(VII) ions is commonly called perchloric acid, HClO4. It is a very strong acid and is commonly classed as a superacid as it is a stronger acid than sulfuric acid. (i) State how the Ka value of a stronger acid will compare with that of a weaker acid, giving a reason. [1] (ii) The concentration of 25.0 cm3 of an aqueous solution of perchloric acid can be found by adding an excess of calcium carbonate and then measuring the amount of carbon dioxide gas released. This can be done by either: • measuring the volume of the gas released using a gas syringe, or • measuring the mass lost as the carbon dioxide is released by placing the reaction flask on a digital balance. Joe uses the first method to study his sample of perchloric acid and finds that the reaction releases 87 cm3 of carbon dioxide at 30 °C and 1 atm pressure. Heledd uses the second method and finds that the mixture loses a total of 0.1533 g during the reaction. © WJEC CBAC Ltd. 22 (1410U30-1) Turn over. 23 Examiner only © WJEC CBAC Ltd. I. Find the number of moles of carbon dioxide released in each experiment and hence state whether the two solutions provided to Joe and Heledd are of the same concentration. [4] You must show your working and give your answers to appropriate numbers of significant figures. Moles of CO2 using Joe’s method = ....................................................... mol Moles of CO2 using Heledd’s method = ....................................................... mol II. Suggest, from the data provided, which of the two methods will give better results. Give a reason for your suggestion. [1] III. Isha suggests using excess magnesium in place of calcium carbonate for Joe and Heledd’s experiments. This will produce hydrogen gas in place of carbon dioxide. Explain which of the two methods will suffer the greater loss in accuracy. [2] 23 Examiner only 24 (1410U30-1) © WJEC CBAC Ltd. (iii) One common salt of perchloric acid is ammonium perchlorate. Suggest a pH value for a solution of ammonium perchlorate. Give a reason for your answer. [2] END OF PAPER 24 24 (1410U30-1) Turn over. 25 25 © WJEC CBAC Ltd. Examiner only Question number Additional page, if required. Write the question number(s) in the left-hand margin. 26 (1410U30-1) 26 © WJEC CBAC Ltd. Question number Additional page, if required. Write the question number(s) in the left-hand margin. Examiner only BLANK PAGE PLEASE DO NOT WRITE ON THIS PAGE (1410U30-1) 27 27 © WJEC CBAC Ltd. BLANK PAGE PLEASE DO NOT WRITE ON THIS PAGE (1410U30-1) 28 28 © WJEC CBAC Ltd.